Make: More Electronics

Chapter 18. Experiment 18: Time to Switch

Figure 18-1 introduces a new concept. It shows how each logic gate can be emulated by a pair of plain, ordinary switches, and each switch can be considered as an input to the gate.

When a switch is pressed, it’s the same as a logic input being high. When it is not pressed, this is the same as a logic input being low. Thus, in the illustration, the top pair of switches gives a high output only if the left switch AND the right switch are both pressed. Just like an AND gate!

Although—not quite. An AND gate output is either high or low. When the switches are open in Figure 18-1, the output is an open circuit. It will be floating, with no defined voltage. We need to add a pulldown resistor to control the state ...